Seeking to combat rising launch costs for U.S. military and spy satellites, senior Senate Democrats are prodding the Pentagon to give Space Exploration Technologies Corp., headed by businessman Elon Musk, its first opportunity to compete for such contracts.

Outlines of that effort emerged Wednesday at a Senate defense appropriations subcommittee hearing, with two high-ranking Democrats challenging prices charged by a joint venture between Lockheed Martin Corp. and Boeing Co., currently the Air Force's sole launch provider for big satellites.

Eight years after the Pentagon effectively gave the venture a monopoly to blast national-security satellites into orbit, congressional investigators told the panel overall launch costs have more than doubled to $1.6 billion annually.

In extreme cases, the total bill for blasting a particularly heavy and complex Pentagon satellite into a challenging orbit can be around $500 million, according to industry officials.

While average launch costs run in the $300 million to $380 million range, according to these officials and congressional investigators, that still amounts to a significant long-term financial burden at a time of shrinking military budgets and escalating pressure to keep a lid on spending for space projects.

SpaceX, as the Southern California company is commonly called, has said its price to launch a typical Pentagon satellite would be under $100 million.

Against the backdrop of such financial considerations, Wednesday's session also amounted to a dramatic turnabout and personal vindication for Mr. Musk, the former Internet entrepreneur who founded and continues to lead fast-growing SpaceX, based in Hawthorne, Calif.